Process for the preparation of high chloride tabular grain emulsions (II)

ABSTRACT

A process of preparing a radiation sensitive high chloride high aspect ratio tabular grain emulsion is disclosed wherein silver ion is introduced into a gelatino-peptizer dispersing medium containing a stoichiometric excess of chloride ions a chloride ion concentration of less than 0.5 molar and a grain growth modifier of the formula: ##STR1## where Z 2  is --C(R 2 )═ or --N═; 
     Z 3  is --C(R 3 )═ or --N═; 
     Z 4  is --C(R 4 )═ or --N═; 
     Z 5  is --C(R 5 )═ or --N═; 
     Z 6  is --C(R 6 )═ or --N═; 
     with the proviso that no more than one of Z 4 , Z 5  and Z 6  is --N═; 
     R 2  is H, NH 2  or CH 3  ; 
     R 3 , and R 4  and R 5  are independently selected, R 3  and R 5  being hydrogen, hydroxy, halogen, amino or hydrocarbon and R 4  being hydrogen, halogen or hydrocarbon, each hydrocarbon moiety containing from 1 to 7 carbon atoms; and 
     R 6  is H or NH 2 .

FIELD OF THE INVENTION

The invention relates to the precipitation of radiation sensitive silverhalide emulsions useful in photography.

BACKGROUND OF THE INVENTION

Radiation sensitive silver halide emulsions containing one or acombination of chloride, bromide and iodide ions have been longrecognized to be useful in photography. Each halide ion selection isknown to impart particular photographic advantages. Although known andused for many years for selected photographic applications, the morerapid developability and the ecological advantages of high chlorideemulsions have provided an impetus for employing these emulsions over abroader range of photographic applications. As employed herein the term"high chloride emulsion" refers to a silver halide emulsion containingat least 50 mole percent chloride and less than 5 mole percent iodide,based on total silver.

During the 1980's a marked advance took place in silver halidephotography based on the discovery that a wide range of photographicadvantages, such as improved speed-granularity relationships, increasedcovering power both on an absolute basis and as a function of binderhardening, more rapid developability, increased thermal stability,increased separation of native and spectral sensitization impartedimaging speeds, and improved image sharpness in both mono- andmulti-emulsion layer formats, can be realized by increasing theproportions of selected tabular grain populations in photographicemulsions.

The various photographic advantages were associated with achieving highaspect ratio tabular grain emulsions. As herein employed and as normallyemployed in the art, the term "high aspect ratio tabular grain emulsion"has been defined as a photographic emulsion in which tabular grainshaving a thickness of less than 0.3 μm and an average aspect ratio ofgreater than 8:1 account for at least 50 percent of the total grainprojected area of emulsion. Aspect ratio is the ratio of tabular graineffective circular diameter (ECD), divided by tabular grain thickness(t).

Although the art has succeeded in preparing high chloride tabular grainemulsions, the inclusion of high levels of chloride as opposed tobromide, alone or in combination with iodide, has been difficult. Thebasic reason is that tabular grains are produced by incorporatingparallel twin planes in grains grown under conditions favoring {111}crystal faces. The most prominent feature of tabular grains are theirparallel {111} major crystal faces.

To produce successfully a high chloride tabular grain emulsion twoobstacles must be overcome. First, conditions must be found thatincorporate parallel twin planes into the grains. Second, the strongpropensity of silver chloride to produce {100} crystal faces must beovercome by finding conditions that favor the formation of {111} crystalfaces.

Wey U.S. Pat. No. 4,399,215 produced the first silver chloride highaspect ratio (ECD/t>8) tabular grain emulsion. An ammoniacal double-jetprecipitation technique was employed. The tabularity of the emulsionswas not high compared to contemporaneous silver bromide and bromoiodidetabular grain emulsions because the ammonia thickened the tabulargrains. A further disadvantage was that significant reductions intabularity occurred when bromide and/or iodide ions were included in thetabular grains.

Wey et al U.S. Pat. No. 4,414,306 developed a process for preparingsilver chlorobromide emulsions containing up to 40 mole percent chloridebased on total silver. This process of preparation has not beensuccessfully extended to high chloride emulsions.

Maskasky U.S. Pat. No. 4,400,463 (hereinafter designated Maskasky I)developed a strategy for preparing a high chloride, high aspect ratiotabular grain emulsion capable of tolerating significant inclusions ofthe other halides. The strategy was to use a particularly selectedsynthetic polymeric peptizer in combination with a grain growth modifierhaving as its function to promote the formation of {111} crystal faces.Adsorbed aminoazaindenes, preferably adenine, and iodide ions weredisclosed to be useful grain growth modifiers. The principaldisadvantage of this approach has been the necessity of employing asynthetic peptizer as opposed to the gelatino-peptizers almostuniversally employed in photographic emulsions.

This work has stimulated further investigations of grain growthmodifiers for preparing tabular grain high chloride emulsions, asillustrated by Takada et al U.S. Pat. No. 4,783,398, which employsheterocycles containing a divalent sulfur ring atom; Nishikawa et alU.S. Pat. No. 4,952,491, which employs spectral sensitizing dyes anddivalent sulfur atom containing heterocycles and acyclic compounds; andIshiguro et al U.S. Pat. No. 4,983,508, which employs organicbis-quaternary amine salts.

Maskasky U.S. Pat. No. 4,713,323 (hereinafter designated Maskasky II),continuing to use aminoazaindene growth modifiers, particularly adenine,discovered that tabular grain high chloride emulsions could be preparedby running silver salt into a dispersing medium containing at least a0.5 molar concentration of chloride ion and an oxidizedgelatino-peptizer. An oxidized gelatino-peptizer is a gelatino-peptizertreated with a strong oxidizing agent to modify by oxidation (andeliminate or reduce as such) the methionine content of the peptizer.Maskasky II taught to reduce the methionine content of the peptizer to alevel of less than 30 micromoles per gram. King et al U.S. Pat. No.4,942,120 is essentially cumulative, differing only in that methioninewas modified by alkylation.

While Maskasky II overcame the synthetic peptizer disadvantage ofMaskasky I, the requirement of a chloride ion concentration of at least0.5 molar in the dispersing medium during precipitation presentsdisadvantages. At the elevated temperatures typically employed foremulsion precipitations using gelatino-peptizers, the high chloride ionconcentrations corrode the stainless steel vessels used for thepreparation of photographic emulsions. Additionally, the high chlorideion concentrations increase the amount of emulsion washing requiredafter precipitation, and disposal of the increased levels of chlorideion represents increased consumption of materials and an increasedecological burden.

Tufano et al U.S. Pat. No. 4,804,621 disclosed a process for preparinghigh aspect ratio tabular grain high chloride emulsions in agelatino-peptizer. Tufano et al taught that over a wide range ofchloride ion concentrations ranging from pCl 0 to 3 (1 to 1×10⁻³ M)4,6-diaminopyrimidines satisfying specific structural requirements wereeffective growth modifiers for producing high chloride tabular grainemulsions. Tufano et al specifically required that the followingstructural formula be satisfied: ##STR2## wherein Z is C or N; R₁, R₂and R₃, which may be the same or different, are H or alkyl of 1 to 5carbon atoms; Z is C, R₂ R₃ when taken together can be --CR₄ ═CR₅ -- or--CR₄ ═N--, wherein R₄ and R₅, which may be the same or different are Hor alkyl of 1 to 5 carbon atoms, with the proviso that when R₂ and R₃taken together form the --CR₄ ═N-- linkage, --CR₄ ═ must be joined to Z.Tufano et al also contemplated salts of the formula compound. Tufano etal demonstrated the failure of adenine as a growth modifier. Thus,Tufano et al discourages the selection of heterocycles for use as graingrowth modifiers that lack two primary or secondary amino ringsubstituents in the indicated relationship to the pyrimidine ringnitrogen atoms and those compounds that contain a nitrogen atom linkedto the 5-position of the pyrimidine ring.

RELATED PATENT APPLICATIONS

Maskasky U.S. Ser. No. 763,382, concurrently filed, now abandoned, andcommonly assigned, titled IMPROVED PROCESS FOR THE PREPARATION OF HIGHCHLORIDE TABULAR GRAIN EMULSIONS (I), (hereinafter designated MaskaskyIII) discloses a process for preparing a high chloride tabular grainemulsion in which silver ion is introduced into a gelatino-peptizerdispersing medium containing a stoichiometric excess of chloride ions ofless than 0.5 molar, a pH of at least 4.6, and a4,6-di(hydroamino)-5-aminopyrimidine grain growth modifier.

Maskasky et al U.S. Ser. No. 763,013, concurrently filed and commonlyassigned, titled IMPROVED PROCESS FOR THE PREPARATION OF HIGH CHLORIDETABULAR GRAIN EMULSIONS (III), (hereinafter designated Maskasky et al)discloses a process for preparing a high chloride tabular grain emulsionin which silver ion is introduced into a gelatino-peptizer dispersingmedium containing a stoichiometric excess of chloride ions of less than0.5 molar and a grain growth modifier of the formula: ##STR3## where Z⁸is --C(R⁸)═ or --N═;

R⁸ is H, NH₂ or CH₃ ; and

R¹ is hydrogen or a hydrocarbon containing from 1 to 7 carbon atoms.

Maskasky U.S. Ser. No. 763,030, concurrently filed and commonlyassigned, titled ULTRATHIN HIGH CHLORIDE TABULAR GRAIN EMULSIONS,(hereinafter designated Maskasky IV) discloses a high chloride tabulargrain emulsion in which greater than 50 percent of the total grainprojected area is accounted for by ultrathin tabular grains having athickness of less than 360 {111} crystal lattice planes. A {111} crystalface stabilizer is adsorbed to the major faces of the ultrathin tabulargrains.

SUMMARY OF THE INVENTION

In one aspect, this invention is directed to a process of preparing aradiation sensitive high aspect ratio tabular grain emulsion, whereintabular grains of less than 0.3 μm in thickness and an average aspectratio of greater than 8:1 account for greater than 50 percent of thetotal grain projected area, the tabular grains containing at least 50mole percent chloride, based on silver, comprising introducing silverion into a gelatino-peptizer dispersing medium containing astoichiometric excess of chloride ions with respect to the silver ionsfurther characterized by a chloride ion concentration of less than 0.5molar and a grain growth modifier of the formula: ##STR4## where Z² is--C(R²)═ or --N═;

Z³ is --C(R³)═ or --N═;

Z⁴ is --C(R⁴)═ or --N═;

Z⁵ is --C(R⁵)═ or --N═;

Z⁶ is --C(R⁶)═ or --N═;

with the proviso that no more than one of Z⁴, Z⁵ and Z⁶ is --N═;

R² is H, NH₂ or CH₃ ;

R³, R⁴ and R⁵ are independently selected, R³ and R⁵ being hydrogen,hydroxy, halogen, amino or hydrocarbon and R⁴ being hydrogen, halogen orhydrocarbon, each hydrocarbon moiety containing from 1 to 7 carbonatoms; and

R⁶ is H or NH₂.

It has been discovered quite unexpectedly that a novel class of graingrowth modifiers are capable of producing high chloride tabular grainemulsions at unexpectedly low stoichiometric levels of excess chlorideion. The lowered stoichiometric excess of chloride ion avoids thecorrosion, increased washing, materials consumption and ecologicalburden concerns inherent in the Maskasky II process. The disadvantage ofMaskasky I of requiring a synthetic peptizer is also avoided. At thesame time, in contradiction of the molecular structure taught by Tufanoet al to be essential, a whole new class of grain growth modifiers arerecognized to be useful, including many that are of ready commercialavailability. Thus, the process of the invention provides a practicaland attractive preparation of high chloride tabular grain emulsions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are scanning electron photomicrographs of an emulsionprepared according to the invention. In FIG. 1 the emulsion is viewedperpendicular to the support, and in FIG. 2 the emulsion is viewed at adeclination of 60° from the perpendicular and at high level ofmagnification.

DESCRIPTION OF PREFERRED EMBODIMENTS

In preferred embodiments the processes of preparing high chloride highaspect ratio tabular grain emulsions of this invention employ a novelclass of grain growth modifiers satisfying the formula: ##STR5## whereZ² is --C(R²)═ or --N═;

Z³ is --C(R³)═ or --N═;

Z⁴ is --C(R⁴)═ or --N═;

Z⁵ is --C(R⁵)═ or --N═;

Z⁶ is --C(R⁶)═ or --N═;

with the proviso that no more than one of Z⁴, Z⁵ and Z⁶ is --N═;

R² is H, NH₂ or CH₃ ;

R³, R⁴ and R⁵ are independently selected, R³ and R⁵ being hydrogen,hydroxy, halogen, amino or hydrocarbon and R⁴ being hydrogen, halogen orhydrocarbon, each hydrocarbon moiety containing from 1 to 7 carbonatoms; and

R⁶ is H or NH₂.

The grain growth modifiers of formula I in none of their various relatedforms permit a primary or secondary amino substituent R⁴, whereas Tufanoet al requires such an amino substitution in this position. The presentinvention, in fact, requires no amino substituent, allowing both R⁴ andZ⁴ to take forms entirely excluded by Tufano et al. Another distinctionover the grain growth modifiers of Tufano et al, present in many of themost practical forms of the invention, lies in the presence of anitrogen atom attached to the six membered ring at the Z³ position.Still another distinction from Tufano et al is present when Z⁶ is --N═.

In preferred forms the grain growth modifiers of formula I complete aheterocyclic nucleus chosen from the group consisting of 7-azaindole;4,7-diazaindole; 5,7-diazaindole; 6,7-diazaindole; purine;4-azabenzimidazole; 4,7-diazabenzimidazole; 4-azabenzotriazole;4,7-diazabenzotriazole; and 1,2,5,7-tetraazaindene.

When the grain growth modifier is chosen to have a 7-azaindole nucleus,the structure of the grain growth modifier is as shown in the followingformula ##STR6##

When the grain growth modifier is chosen to have a 4,7-diazaindolenucleus, the structure of the grain growth modifier is as shown in thefollowing formula: ##STR7##

When the grain growth modifier is chosen to have a 5,7-diazaindolenucleus, the structure of the grain growth modifier is as shown in thefollowing formula: ##STR8##

When the grain growth modifier is chosen to have a 6,7-diazaindolenucleus, the structure of the grain growth modifier is as shown in thefollowing formula: ##STR9##

When the grain growth modifier is chosen to have a purine nucleus, thestructure of the grain growth modifier is as shown in the followingformula: ##STR10##

When the grain growth modifier is chosen to have a 4-azabenzimidazolenucleus, the structure of the grain growth modifier is as shown in thefollowing formula: ##STR11##

With the inclusion of an additional nitrogen atom to the ring structure,the 4-azabenzimidazole can become a 4,7-diazabenzimidazole of theformula: ##STR12##

When the grain growth modifier is chosen to have a 4-azabenzotriazolenucleus, the structure of the grain growth modifier is as shown in thefollowing formula: ##STR13##

With the inclusion of an additional nitrogen atom to the ring structure,the 4-azabenzotriazole can become a 4,7-diazabenzotriazole of theformula: ##STR14##

When the grain growth modifier is chosen to have a1,2,5,7-tetraazaindene nucleus, the structure of the grain growthmodifier is as shown in the following formula: ##STR15##

No substituents of any type are required on the ring structures offormulae I to XI. Thus, each of R², R³, R⁴, R⁵ and R⁶ (hereinaftercollectively referred to as R²⁻⁶) can in each occurrence be hydrogen. Inaddition to hydrogen R²⁻⁶ can (except for R⁴) include an aminosubstituent. When R² and R⁶ are amino substituents they are primaryamino substituents. When R³ and R⁵ are amino substituents, they can bechosen from among primary, secondary or tertiary amino substituents.Primary amino substituents can be represented by the formula --NH₂ ; thesecondary amino substituents can be represented by the formula --NHR;and the tertiary amino substituents can be represented by the formula--NR₂, where R in each occurrence is preferably a hydrocarbon of from 1to 7 carbon atoms. R² can in addition include a sterically compacthydrocarbon substituent, such as methyl. R³, R⁴ and R⁵ can independentlyin each occurrence additionally include halogen or hydrocarbon carbonsubstituents of from 1 to 7 carbon atoms. R³ and R⁵ can additionallyinclude a hydroxy substituent. Each hydrocarbon moiety is preferably analkyl group--e.g., methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl,t-butyl, etc., although other hydrocarbons, such as cyclohexyl orbenzyl, are contemplated. To increase grain growth modifier solubilitythe hydrocarbon groups can, in turn, be substituted with polar groups,such as hydroxy, sulfonyl or amino groups, or the hydrocarbon groups canbe substituted with other groups that do not materially modify theirproperties (e.g., a halo substituent), if desired.

An aqueous gelatino-peptizer dispersing medium is present duringprecipitation. Gelatino-peptizers include gelatin--e.g., alkali-treatedgelatin (cattle bone and hide gelatin) or acid-treated gelatin (pigskingelatin) and gelatin derivatives--e.g., acetylated gelatin, phthalatedgelatin, and the like.

The process of the invention is not restricted to use withgelatino-peptizers of any particular methionine content. That is,gelatino-peptizers with all naturally occurring methionine levels areuseful. It is, of course, possible, though not required, to reduce oreliminate methionine, as taught by Maskasky II or King et al, both citedabove and here incorporated by reference.

During the precipitation of photographic silver halide emulsions thereis always a slight stoichiometric excess of halide ion present. Thisavoids the possibility of excess silver ion being reduced to metallicsilver and resulting in photographic fog. It is a significant advantageof this invention that the stoichiometric excess of chloride ion in thedispersing medium can be maintained at a chloride ion concentrationlevel of less than 0.5M while still obtaining a high aspect ratiotabular grain emulsion. It is generally preferred that the chloride ionconcentration in the dispersing medium be less than 0.2M and, optimally,equal to or less than 0.1M.

The advantages of limiting the stoichiometric excess of chloride ionpresent in the reaction vessel during precipitation include (a)reduction of corrosion of the equipment (the reaction vessel, thestirring mechanism, the feed jets, etc.), (b) reduced consumption ofchloride ion, (c) reduced washing of the emulsion after preparation, and(d) reduced chloride ion in effluent. It has also been observed thatreduction in the chloride ion excess contributes to obtaining thinnertabular grains.

The grain growth modifiers of the invention are effective over a widerange of pH levels conventionally employed during the precipitation ofsilver halide emulsions. It is contemplated to maintain the dispersingmedium within conventional pH ranges for silver halide precipitation,typically from 3 to 9, while the tabular grains are being formed, with apH range of 4.5 to 8 being in most instances preferred. Within these pHranges optimum performance of individual grain growth modifiers can beobserved as a function of their specific structure. A strong mineralacid, such as nitric acid or sulfuric acid, or a strong mineral base,such as an alkali hydroxide, can be employed to adjust pH within aselected range. When a basic pH is to be maintained, it is preferred notto employ ammonium hydroxide, since it has the unwanted effect of actingas a ripening agent and is known to thicken tabular grains. However, tothe extent that thickening of the tabular grains does not exceed the 0.3μm thickness limit, ammonium hydroxide or other conventional ripeningagents (e.g., thioether or thiocyanate ripening agents) can be presentwithin the dispersing medium.

Any convenient conventional approach of monitoring and maintainingreplicable pH profiles during repeated precipitations can be employed(e.g., refer to Research Disclosure Item 308,119, cited below).Maintaining a pH buffer in the dispersing medium during precipitationarrests pH fluctuations and facilitates maintenance of pH withinselected limited ranges. Exemplary useful buffers for maintainingrelatively narrow pH limits within the ranges noted above include sodiumor potassium acetate, phosphate, oxalate and phthalate as well astris(hydroxymethyl)aminomethane.

In forming high chloride high aspect ratio tabular grain emulsions,tabular grains containing at least 50 mole percent chloride, based onsilver, and having a thickness of less than 0.3 μm must account forgreater than 50 percent of the total grain projected area. In preferredemulsions the tabular grains having a thickness of less than 0.2 μmaccount for at least 70 percent of the total grain projected area.

For tabular grains to satisfy the projected area requirement it isnecessary first to induce twinning in the grains as they are beingformed, since only grains having two or more parallel twin planes willassume a tabular form. Second, after twinning has occurred, it isnecessary to restrain precipitation onto the major {111} crystal facesof the tabular grains, since this has the effect of thickening thegrains. The grain growth modifiers employed in the practice of thisinvention are effective during precipitation to produce an emulsionsatisfying both the tabular grain thickness and projected areaparameters noted above.

It is believed that the effectiveness of the grain growth modifiers toinduce twinning during precipitation results from the spacing of therequired nitrogen atoms in the fused five and six membered heterocyclicrings and their ability to form silver salts. This can be betterappreciated by reference to the following structure: ##STR16## C. Cagnonet al, Inorganic Chem., 16:2469 (1977) reports a silver salt satisfyingformula XII and provides bond lengths establishing the spacing betweenthe adjacent silver atoms of the formula. Based on the crystal structureof silver chloride as revealed by X-ray diffraction it is believed thatthe resulting spacing between the silver ions is much closer to thenearest permissible spacing of silver ions in next adjacent {111} silverion crystal lattice planes separated by a twin plane than the nearestspacing of silver ions in next adjacent {111} silver ion crystal latticeplanes not separated by a twin plane. Thus, when one of the silver ionsshown above is positioned during precipitation in a {111} silver ioncrystal lattice plane, assuming a sterically compatible location (e.g.,an edge, pit or coign position) is occupied, the remaining of the silverions shown above favors a position in the next {111} silver ion crystallattice plane that is permitted only if twinning occurs. The remainingsilver atom of the growth modifier (together with other similarlysituated growth modifier silver ions) acts to seed (enhance theprobability of) a twin plane being formed and growing across the {111}crystal lattice face, thereby providing a permanent crystal featureessential for tabular grain formation.

It is, of course, also important that any ring substituents forming apart of Z² and Z⁶ next adjacent the ring nitrogen shown in formula XIIbe chosen to minimize any steric hindrance that would prevent the silverions from having ready access to the {111} crystal lattice planes asthey are being formed. A further consideration is to avoid substituentsforming a part of Z² and Z⁶ at the ring positions next adjacent the ringnitrogen shown that are strongly electron withdrawing, since thiscreates competition between the silver ions and the adjacent ringposition for the π electrons of the nitrogen atoms. When Z² and Z⁶ are--N═ or --CH═, an optimum structure for silver ion placement in thecrystal lattice exists. When Z² and Z⁶ represent --C(R²)═ or --C(R⁶),respectively, where R² and R⁶ are compact substituents, as describedabove, twin plane formation is readily realized.

In formula XII the --Z³ ═, --Z⁴ ═ and --Z⁵ ═ ring positions are notshown, since, apart from being necessary to impart aromaticity, thesering positions and their substituents are not viewed as significantlyinfluencing twin plane formation. Unlike substituents R₂ and R⁶,substituents R³, R⁴ and R⁵ are sufficiently removed from the requiredring nitrogen atoms to have minimal, if any, steric influence on silverion deposition.

In addition to selecting substituents for their role in twin planeformation, they must also be selected for their compatibility withpromoting the formation of {111} crystal faces during precipitation. Byselecting substituents as described above the emergence of {100}, {110}and higher index crystal plane faces of the types described by MaskaskyU.S. Pat. Nos. 4,643,966, 4,680,254, 4,680,255, 4,680,256 and 4,724,200,is avoided. In those instances in which a second grain growth modifieris relied upon to assure emergence of {111} crystal faces duringprecipitation, a broadened selection of substituents not affecting twinplane formation is specifically contemplated.

It is generally recognized that introducing twin planes in the grains ata very early stage in their formation offers the capability of producingthinner tabular grains than can be achieved when twinning is delayed.For this reason it is usually preferred that the conditions within thedispersing medium prior to silver ion introduction at the outset ofprecipitation be chosen to favor twin plane formation. To facilitatetwin plane formation it is contemplated to incorporate the grain growthmodifier in the dispersing medium prior to silver ion addition in aconcentration of at least 2×10⁻⁴ M, preferably at least 5×10⁻⁴ M, andoptimally at least 7×10⁻⁴ M. Generally little increase in twinning canbe attributed to increasing the initial grain growth modifierconcentration in the dispersing medium above 0.01M. Higher initial graingrowth modifier concentrations up to 0.05M, 0.1M or higher are notincompatible with the twinning function. The maximum growth modifierconcentration in the dispersing medium is often limited by itssolubility. It is contemplated to introduce into the dispersing mediumgrowth modifier in excess of that which can be initially dissolved. Anyundissolved growth modifier can provide a source of additional growthmodifier solute during precipitation, thereby stabilizing growthmodifier concentrations within the ranges noted above. It is preferredto avoid quantities of grain growth modifier in excess of those observedto control favorably tabular grain parameters.

Once a stable multiply twinned grain population has been formed withinthe dispersing medium, the primary, if not exclusive, function the graingrowth modifier is called upon to perform is to restrain precipitationonto the major {111} crystal faces of the tabular grains, therebyretarding thickness growth of the tabular grains. In a well controlledtabular grain emulsion precipitation, once a stable population ofmultiply twinned grains has been produced, tabular grain thicknesses canbe held essentially constant.

The amount of grain growth modifier required to control thickness growthof the tabular grain population is a function of the total grain surfacearea. By adsorption onto the {111} surfaces of the tabular grains thegrain growth modifier restrains precipitation onto the grain faces andshifts further growth of the tabular grains to their edges.

The benefits of this invention can be realized using any amount of graingrowth modifier that is effective to retard thickness growth of thetabular grains. It is generally contemplated to have present in theemulsion during tabular grain growth sufficient grain growth modifier toprovide a monomolecular adsorbed layer over at least 25 percent,preferably at least 50 percent, of the total {111} grain surface area ofthe emulsion grains. Higher amounts of adsorbed grain growth modifierare, of course, feasible. Adsorbed grain growth modifier coverages of 80percent of monomolecular layer coverage or even 100 percent arecontemplated. In terms of tabular grain thickness control there is nosignificant advantage to be gained by increasing grain growth modifiercoverages above these levels. Any excess grain growth modifier thatremains unadsorbed is normally depleted in post-precipitation emulsionwashing.

Prior to introducing silver salt into the dispersing medium at theoutset of the precipitation process, no grains are present in thedispersing medium, and the initial grain growth modifier concentrationsin the dispersing medium are therefore more than adequate to provideinitially the monomolecular coverage levels noted above. As tabulargrain growth progresses it is a simple matter to add grain growthmodifier, as needed, to maintain monomolecular coverages at desiredlevels, based on knowledge of amount of silver ion added and thegeometrical forms of the grains being grown. If, as noted above, graingrowth modifier has been initially added in excess of its solubilitylimit, undissolved grain growth modifier can enter solution asadditional dispersing medium is introduced during grain growth. This canreduce or even eliminate any need to add grain growth modifier to thereaction vessel as grain growth progresses.

The grain growth modifiers described above are capable of use duringprecipitation as the sole grain growth modifier. That is, these graingrowth modifiers are capable of influencing both twinning and tabulargrain growth to provide high chloride high aspect ratio tabular grainemulsions.

It has been discovered that improvements in precipitation can berealized by employing a combination of grain growth modifiers in whichthe more tightly adsorbed of the grain growth modifiers is employed fortabular grain thickness growth reduction while the less tightly adsorbedof the grain growth modifiers is employed for twinning. Different graingrowth modifiers of this invention can be employed in combination onthis basis, with the less tightly adsorbed grain growth modifier beingemployed during grain twinning and the more tightly adsorbed graingrowth modifier being present during grain growth following twinning.

Instead of employing a grain growth modifier of this invention toperform each of the twinning and tabular grain thickness controlfunctions, it is possible to employ another growth modifier to performone of these two functions.

It is specifically contemplated to employ during twinning or graingrowth a grain growth modifier of the following structure: ##STR17##wherein Z is C or N; R₁, R₂ and R₃, which may be the same or different,are H or alkyl of 1 to 5 carbon atoms; Z is C, R₂ and R₃ when takentogether can be --CR₄ ═CR⁵ -- or --CR₄ ═N--, wherein R₄ and R₅, whichmay be the same or different are H or alkyl of 1 to 5 carbon atoms, withthe proviso that when R₂ and R₃ taken together form the --CR₄ ═N--linkage, --CR₄ ═ must be joined to Z. Grain growth modifiers of thistype and conditions for their use are disclosed by Tufano et al, citedabove, the disclosure of which is here incorporated by reference.

It is also contemplated to employ during grain twinning or grain growthfollowing twinning a grain growth modifier of the type disclosed byMaskasky III, cited above. These grain growth modifiers are effectivewhen the dispersing medium is maintained at a pH in the range of from4.6 to 9 (preferably 5.0 to 8) and contains a stoichiometric excess ofchloride ions of less than 0.5 molar. These grain growth modifiers are4,6-di(hydro-amino)-5-aminopyrimidine grain growth modifiers, withpreferred compounds satisfying the formula: ##STR18## where N⁴, N⁵ andN⁶ are amino moieties independently containing hydrogen or hydrocarbonsubstituents of from 1 to 7 carbon atoms, with the proviso that the N⁵amino moiety can share with each or either of N⁴ and N⁶ a commonhydrocarbon substituent completing a five or six member heterocyclicring. The grain growth modifiers of this formula when present duringgrain twinning are capable of producing ultrathin tabular grainemulsions.

Another class of grain growth modifier useful during grain twinning orgrowth under similar conditions as the grain growth modifiers of formulaVI are the xanthine type grain growth modifiers of Maskasky et al, citedabove. These grain growth modifiers are represented by the formula:##STR19## where Z⁸ is --C(R⁸)═ or --N═;

R⁸ is H, NH₂ or CH₃ ; and

R¹ is hydrogen or a hydrocarbon containing from 1 to 7 carbon atoms.

Still another type of grain growth modifier contemplated for use duringgrain growth is iodide ion. The use of iodide ion as a grain growthmodifier is taught by Maskasky I, the disclosure of which is hereincorporated by reference.

In Maskasky U.S. Ser. No. 623,839, filed Dec. 7, 1990, AN IMPROVEDPROCESS FOR THE PREPARATION OF HIGH CHLORIDE TABULAR GRAIN EMULSIONS,commonly assigned, (hereinafter referred to as Maskasky VI) it is taughtto maintain a concentration of thiocyanate ions in the dispersing mediumof from 0.2 to 10 mole, based on total silver introduced, to produce ahigh chloride tabular grain emulsion. It is here contemplated to utilizethiocyanate ion in a similar manner to control tabular grain growth.However, whereas Maskasky VI employs a 0.5M concentration of chlorideion in the dispersing medium, the presence of the4,6-di(hydroamino)-5-amino-pyrimidine grain growth modifier in thedispersing medium at the outset of precipitation allows lower chlorideion levels to be present in the dispersing medium, as described above.The thiocyanate ion can be introduced into the dispersing medium as anyconvenient soluble salt, typically an alkali or alkaline earththiocyanate salt. When the dispersing medium is acidic (i.e., the pH isless than 7.0) the counter ion of the thiocyanate salt can be ammoniumion, since ammonium ion releases an ammonia ripening agent only underalkaline conditions. Although not preferred, an ammonium counter ion isnot precluded under alkaline conditions, since, as noted above, ripeningcan be tolerated to the extent that the 0.3 μm thickness limit of thetabular grains is not exceeded.

In addition to or in place of the preferred growth modifiers for use incombination with any of the growth modifiers of this invention it iscontemplated to employ other conventional growth modifiers, such any ofthose disclosed by Takada et al, Nishikawa et al, and Ishiguro et al,cited above and here incorporated by reference.

Since silver bromide and silver iodide are markedly less soluble thansilver chloride, it is appreciated that bromide and/or iodide ions, ifintroduced into the dispersing medium, are incorporated into the grainsin the presence to the chloride ions. The inclusion of bromide ions ineven small amounts has been observed to improve the tabularities of theemulsions. Bromide ion concentrations of up to 50 mole percent, based ontotal silver are contemplated, but to increase the advantages of highchloride concentrations it is preferred to limit the presence of otherhalides so that chloride accounts for at least 80 mole percent, based onsilver, of the completed emulsion. Iodide can be also incorporated intothe grains as they are being formed. It is preferred to limit iodideconcentrations to 2 mole percent or less based on total silver. Thus,the process of the invention is capable of producing high chloridetabular grain emulsions in which the tabular grains consist essentiallyof silver chloride, silver bromochloride, silver iodochloride or silveriodobromochloride, where the halides are designated in order ofascending concentrations.

Either single-jet or double-jet precipitation techniques can be employedin the practice of the invention, although the latter is preferred.Grain nucleation can occur before or instantaneously following theaddition of silver ion to the dispersing medium. While sustained orperiodic subsequent nucleation is possible, to avoid polydispersity andreduction of tabularity, once a stable grain population has beenproduced in the reaction vessel, it is preferred to precipitateadditional silver halide onto the existing grain population.

In one approach silver ion is first introduced into the dispersingmedium as an aqueous solution, such as a silver nitrate solution,resulting in instantaneous grain nuclei formation followed immediatelyby addition of the growth modifier to induce twinning and tabular graingrowth. Another approach is to introduce silver ion into the dispersingmedium as preformed seed grains, typically as a Lippmann emulsion havingan ECD of less than 0.05 μm. A small fraction of the Lippmann grainsserve as deposition sites while the remaining Lippmann grains dissociateinto silver and halide ions that precipitate onto grain nuclei surfaces.Techniques for using small, preformed silver halide grains as afeedstock for emulsion precipitation are illustrated by Mignot U.S. Pat.No. 4,334,012; Saito U.S. Pat. No. 4,301,241; and Solberg et al U.S.Pat. No. 4,433,048, the disclosures of which are here incorporated byreference. In still another approach, immediately following silverhalide seed grain formation within or introduction into a reactionvessel, a separate step is provided to allow the initially formed grainnuclei to ripen. During the ripening step the proportion of untwinnedgrains can be reduced, thereby increasing the tabular grain content ofthe final emulsion. Also, the thickness and diameter dispersities of thefinal tabular grain population can be reduced by the ripening step.Ripening can be performed by stopping the flow of reactants whilemaintaining initial conditions within the reaction vessel or increasingthe ripening rate by adjusting pH, the chloride ion concentration,and/or increasing the temperature of the dispersing medium. The pH,chloride ion concentration and grain growth modifier selectionsdescribed above for precipitation can be first satisfied from the outsetof silver ion precipitation or during the ripening step.

Except for the distinguishing features discussed above, precipitationaccording to the invention can take any convenient conventional form,such as disclosed in Research Disclosure Vol. 225, January 1983, Item22534; Research Disclosure Vol. 308, December 1989, Item 308,119(particularly Section I); Maskasky I, cited above; Wey et al, citedabove; and Maskasky II, cited above; the disclosures of which are hereincorporated by reference. It is typical practice to incorporate fromabout 20 to 80 percent of the total dispersing medium into the reactionvessel prior to nucleation. At the very outset of nucleation a peptizeris not essential, but it is usually most convenient and practical toplace peptizer in the reaction vessel prior to nucleation. Peptizerconcentrations of from about 0.2 to 10 (preferably 0.2 to 6) percent,based on the total weight of the contents of the reaction vessel aretypical, with additional peptizer and other vehicles typically be addedto emulsions after they are prepared to facilitate coating.

Once the nucleation and growth steps have been performed the emulsionscan be applied to photographic applications following conventionalpractices. The emulsions can be used as formed or further modified orblended to satisfy particular photographic aims. It is possible, forexample, to practice the process of this invention and then to continuegrain growth under conditions that degrade the tabularity of the grainsand/or alter their halide content. It is also common practice to blendemulsions once formed with emulsions having differing graincompositions, grain shapes and/or tabular grain thicknesses and/oraspect ratios.

EXAMPLES

The invention can be better appreciated by reference to the followingexamples.

The mean thickness of tabular grain populations was measured by opticalinterference for mean thicknesses >0.06 μm measuring more than 1000tabular grains.

The terms ECD and t are employed as noted above; r.v. representsreaction vessel; GGM is the acronym for grain growth modifier; TGPAindicates the percentage of the total grain projected area accounted bytabular grain of less than 0.3 μm thickness.

EXAMPLES 1-4 AgCl High Aspect Ratio Tabular Grain Emulsions Made using7-Azaindole as the Grain Growth Modifier Example 1

To a stirred reaction vessel containing 400 mL of a solution at pH 6.0and at 40° C. that was 2% in bone gelatin, 0.040M in NaCl, and 0.20M insodium acetate was added 0.60 mmole of 7-azaindole dissolved in 2 mL ofmethanol. Then a 4M AgNO₃ solution and a 4M NaCl solution were added.The AgNO₃ solution was added at 0.25 mL/min for 4 min then its flow wasstopped for 10 minutes after which time 0.60 mmole of 7-azaindole in 2mL of methanol was added. The AgNO₃ solution flow was resumed at 0.25mL/min for 1 min then the flow rate was accelerated over an additionalperiod of 30 min (20×from start to finish) and finally held constant at5 mL/min until 0.4 mole of AgNO₃ was added. The NaCl solution was addedat a similar rate as needed to maintain a constant pAg of 7.67. When thepH dropped 0.2 units below the starting value of 6.0, the flow ofsolutions was momentarily stopped and the pH was adjusted back to thestarting value. Additional 0.60 mmole portions of 7-azaindole dissolvedin methanol were added when 0.13 and 0.27 mole of AgNO₃ had been added.The results are shown in Table I and in FIGS. 1 and 2.

Example 2

This emulsion was prepared similar to that of Example 1 except that theprecipitation was stopped after 0.27 mole of AgNO₃ had been added. Theresults are given in Table I.

Example 3

This emulsion was prepared similar to that of Example 1 except that theprecipitation was stopped after 0.13 mole of AgNO₃ had been added. Theresults are given in Table I.

EXAMPLE 4

This emulsion was prepared similar to that of Example 2 except thatadditional 7-azaindole was not added after the AgNO₃ solution flow wasresumed. The results are presented in Table I.

                                      TABLE I                                     __________________________________________________________________________                    Pro-                                                                    Final jected                                                                  GGM per                                                                             area as                                                                           Tabular Grain Population                                       AgNO.sub.3                                                                         Ag    fine                                                                              Mean     Mean                                                  added                                                                              (mmole/-                                                                            grains*                                                                           ECD  Mean t                                                                            Aspect                                                                            %                                            Example                                                                            (mole)                                                                             mole) (%) (μm)                                                                            (μm)                                                                           ratio                                                                             TPGA                                         __________________________________________________________________________    1    0.40 6.0   0   1.47 0.086                                                                             17.1                                                                              80                                           2    0.27 6.6   2   1.33 0.083                                                                             16.1                                                                              70                                           3    0.13 9.2   2   0.93 0.077                                                                             12.1                                                                              70                                           4    0.27 4.4   0   1.30 0.089                                                                             14.6                                                                              55                                           __________________________________________________________________________     *ECD < 0.2 μm                                                         

Example 5 High AgCl High Aspect Ratio Tabular Grain Emulsions Made Using7-Azaindole and 4,5,6-Triaminopyrimidine Example 5A

To a stirred reaction vessel containing 400 mL of a solution at pH 6.0and at 40° C. that was 2% in bone gelatin, 0.040M in NaCl and 0.20M insodium acetate was added 0.60 mmole of 7-azaindole dissolve in 2 mL ofmethanol. Then a 4M AgNO₃ solution and a 4M NaCl solution were added.The AgNO₃ solution was added at 0.25 mL/min for 4 minutes, then its flowwas stopped for 10 minutes, after which 0.06 mmole of the second graingrowth modifier, 4,5,6-triaminopyrimidine sulfate dissolved in 25 mL ofdistilled water was added. The AgNO₃ solution flow was resumed at 0.25mL/min for 1 minute, then the flow rate was accelerated over anadditional period of 30 minutes (20× from start to finish) and finallyheld constant for 5 mL/min until 0.4 mole of AgNO₃ was added. The NaClsolution was added at a similar rate as needed to maintain a constantpAg of 7.67. When the pH dropped 0.2 units below the starting value of7.0, the flow of solutions were momentarily stopped, and the pH wasadjusted back to the starting value. The results are given in Table II.

Example 5B

This emulsion was prepared similar to that of Example 5A, except thatthe precipitation was stopped after 0.27 mole of AgNO₃ had been added.The results are presented in Table II.

Example 5C

This emulsion was prepared similar to that of Example 5A, except thatthe precipitation was stopped after 0.13 mole of AgNO₃ had been added.The results are presented in Table II.

Example 6B

This emulsion was prepared similar to that of Example 5A, except thatinstead of the 4,5,6-triaminopyrimidine addition, 0.60 mmole of7-azaindole in 2 mL of methanol added. Also, the precipitation wasstopped after 0.27 mol of AgNO₃ had been added. The results arepresented in Table II.

Example 6C

This emulsion was prepared similar to that of Example 6B, except thatthe precipitation was stopped after 0.13 mol of AgNO₃ had been added.The results are presented in Table II.

                                      TABLE II                                    __________________________________________________________________________    Growth                                                                        modifier* Growth                                                                             Final                                                          in        modifier*                                                                          growth*     Tabular Grain Population                                reaction                                                                           in salt                                                                            modifier per                                                                         AgNO.sub.3                                                                         Mean    Mean                                            vessel                                                                             solution                                                                           Ag mole                                                                              added                                                                              ECD Mean t                                                                            Aspect                                                                            %                                      Example                                                                            (mM) (mM) (mmole/mole                                                                          (mole)                                                                             (μm)                                                                           (μm)                                                                           ratio                                                                             TGPA                                   __________________________________________________________________________    5A   1.5.sup.b                                                                          1.5.sup.t                                                                          1.5.sup.b, 1.5.sup.t                                                                 0.40 2.33                                                                              0.092                                                                             25.4                                                                              75                                     5B   1.5.sup.b                                                                          1.5.sup.5                                                                          2.2.sup.b, 2.2.sup.t                                                                 0.27 2.07                                                                              0.090                                                                             23.0                                                                              80                                     6B   1.5.sup.b                                                                          1.5.sup.b                                                                          4.4.sup.b                                                                            0.27 1.30                                                                              0.089                                                                             14.6                                                                              55                                     5C   1.5.sup.b                                                                          1.5.sup.t                                                                          4.6.sup.b, 4.6.sup.t                                                                 0.13 1.80                                                                              0.083                                                                             21.7                                                                              85                                     6C   1.5.sup.b                                                                          1.5.sup.b                                                                          9.2.sup.b                                                                            0.13 0.90                                                                              0.077                                                                             11.7                                                                              70                                     __________________________________________________________________________     .sup.b = 7Azaindole;                                                          .sup.t = 4,5,6triaminopyrimidine                                         

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:
 1. A process of preparing a radiation sensitive highaspect ratio tabular grain emulsion, wherein tabular grains of less than0.3 μm in thickness and an average aspect ratio of greater than 8:1account for greater than 50 percent of the total grain projected area,said tabular grains containing at least 50 mole percent chloride, basedon silver, comprisingintroducing silver ion into a gelatino-peptizerdispersing medium containing a stoichiometric excess of chloride ionswith respect to the silver ions further characterized by a chloride ionconcentration of less than 0.5 molar and a grain growth modifier of theformula: ##STR20## where Z² is --C(R²)═ or --N═;Z³ is --C(R³)═ or --N═;Z⁴ is --C(R⁴)═ or --N═; Z⁵ is --C(R⁵)═ or --N═; Z⁶ is --C(R⁶)═ or --N═;with the proviso that no more than one of Z⁴, Z⁵ and Z⁶ is --N═; R² isH, NH₂ or CH₃ ; R³, R⁴ and R⁵ are independently selected, R³ and R⁵being hydrogen, hydroxy, halogen, amino or hydrocarbon and R⁴ beinghydrogen, halogen or hydrocarbon, each hydrocarbon moiety containingfrom 1 to 7 carbon atoms; and R⁶ is H or NH₂.
 2. A process according toclaim 1 further characterized in that Z², Z³, Z⁴, Z⁵ and Z⁶ complete aheterocyclic nucleus chosen from the group consisting of 7-azaindole;4,7-diazaindole; 5,7-diazaindole; 6,7-diazaindole; purine;4-azabenzimidazole; 4,7-diazabenzimidazole; 4-azabenzotriazole;4,7-diazabenzotriazole; and 1,2,5,7-tetraazaindene.
 3. A processaccording to claim 1 further characterized in that the grain growthmodifier satisfies the formula: ##STR21##
 4. A process according toclaim 1 further characterized in that the grain growth modifiersatisfies the formula: ##STR22##
 5. A process according to claim 1further characterized in that the grain growth modifier satisfies theformula: ##STR23##
 6. A process according to claim 1 furthercharacterized in that the grain growth modifier satisfies the formula:##STR24##
 7. A process according to claim 1 further characterized inthat the grain growth modifier satisifies the formula: ##STR25##
 8. Aprocess according to claim 1 further characterized in that the graingrowth modifier satisifies the formula: ##STR26##
 9. A process accordingto claim 1 further characterized in that the grain growth modifiersatisifies the formula: ##STR27##
 10. A process according to claim 1further characterized in that the grain growth modifier satisifies theformula: ##STR28##
 11. A process according to claim 1 furthercharacterized in that the grain growth modifier satisifies the formula:##STR29##
 12. A process according to claim 1 further characterized inthat the grain growth modifier satisifies the formula: ##STR30##
 13. Aprocess according to any one of claims claim 3 to 12 inclusive furthercharacterized in that R⁶ and R², where present, are each hydrogen.
 14. Aprocess according to claim 1 further characterized in that thestoichiometric excess of chloride ion is less than 0.2 molar.
 15. Aprocess according to claim 1 further characterized in that the pH canrange up to
 9. 16. A process according to claim 15 further characterizedin that the pH is in the range of from 4.5 to
 8. 17. A process accordingto claim 1 further characterized in that the grain growth modifier ispresent in at least a 7×10⁻⁴ molar concentration.
 18. A processaccording to claim 1 further characterized in that the tabular grainscontain less than 2 mole percent iodide, based on silver.
 19. A processaccording to claim 1 further characterized in that the tabular grainsconsist essentially of silver chloride.
 20. A process according to claim1 further characterized in that the grain growth modifier is presentduring twin plane formation.
 21. A process according to claim 1 furthercharacterized in that the grain growth modifier is the compound7-azaindole.
 22. A process according to claim 1 further characterized inthat the grain growth modifier is employed in combination with a secondgrain growth modifier chosen from the group consisting of:(a) iodideions; (b) thiocyanate ions; (c) a compound of the formula; ##STR31##wherein Z is C or N; R₁, R₂ and R₃, which may be the same or different,are H or alkyl of 1 to 5 carbon atoms; Z is C, R₂ and R₃ when takentogether can be --CR₄ ═CR₅ -- or --CR₄ ═N--, wherein R₄ and R₅, whichmay be the same or different are H or alkyl of 1 or 5 carbon atoms, withthe proviso that when R₂ and R₃ taken together form the --CR₄ ═N--linkage, --CR₄ ═ must be joined to Z; and (d) a compound of the formula:##STR32## where Z⁸ is --C(R⁸)═ or --N═;R⁸ is H, NH₂ or CH₃ ; and R¹ ishydrogen or a hydrocarbon containing from 1 to 7 carbon atoms.